1. Field of the Invention
The present invention relates to internal combustion compression ignition engines and more particularly to such an engine constructed to achieve low emissions of NO.sub.x and hydrocarbons.
2. Description of the Prior Art
There has been a great amount of effort expended in recent years in attempts to provide internal combustion engines for vehicle use with much lower levels of NO.sub.x and hydrocarbon emission than are presently possible. This effort has been dictated in large part by the U.S. Government standards which mandate decreasing levels of such emissions each year for the next several years coupled with the threat of fines and other sanctions if automotive producers fail to meet the prescribed standards.
Although these attempts have taken many forms, and special attention has been given to compression ignition diesel engines, no one to my knowledge has, to date, recognized the relationship between the levels of emission of NO.sub.x and the volume of a pre-combustion chamber with respect to the total combustion chamber volume at TDC of the piston in such engines.
In fact the prior art teaches that the volume of a precombustion chamber should not be more than 50% of the total combustion chamber volume to achieve an efficient engine.
There also has been some recent work which indicates that retarding the initiation of fuel injection in compression ignition engines with respect to presently acceptable standards effectively reduces the emissions of NO.sub.x. The difficulty encountered, however, with engines equipped to retard the initiation of fuel injection has been misfiring and an increase of hydrocarbon emissions at light load. No one to my knowledge has suggested that high compression ratios at light loads and a fuel injection means which variably retards the initiation of fuel injection dependent upon engine speed and/or load will overcome this problem of misfiring and will effectively reduce hydrocarbon emissions even at light load while at the same time producing the low levels of NO.sub.x emissions normally accompanying retarded fuel injection.
Variable compression ratio (VCR) engines are, of course, not new. These are engines in which the compression ratio of the engine is varied to maintain a predetermined maximum combustion chamber pressure. Such engines provide relatively high compression ratios at light loads to improve starting and operating characteristics. The assignee of this application has been quite active in the development of such engines. Again, however, there have been no previous engines of the type in which a pre-combustion chamber is sized to provide more than 50% of the total combustion chamber volume at light loads.